Separating Salt from Water

The problem of separating salt from water has long been solved by forcing the water through a polyamide membrane in a process called reverse osmosis (RO). However, the water can't be disinfected with chlorine because it degrades polyamide material. Now, researchers at Virginia Tech have created a new polymer membrane for RO that will not be degraded by chlorine.

"Our RO materials grew out of our work on proton exchange membrane (PEM) materials used in fuel cells," said James McGrath, University Distinguished Professor of Chemistry at Virginia Tech. "The polymer structure is similar, but PEM materials are treated with a dilute acid and the RO materials are treated with a salt to put them in the neutral form."

Last year, McGrath's group received funding from the Office of Naval Research (ONR) to develop an RO material that would not break down from chlorine. "We have suggested for some time that PEM materials could be used so our students quickly began sending sample materials for testing to Benny D. Freeman, chemical engineer at the University of Texas, Austin. And within a year we had a successful material.

The ONR has expanded the project to add Don Baird, professor of chemical engineering at Virginia Tech, to fabricate the membrane. "The material we created and evaluated in the first year was relatively thick," McGrath said. "To be competitive, it has to be a thin film so the water can pass through quickly - 10 to 100 times thinner than our present samples. That is not trivial but we think we know how to do it."

The Virginia Tech research group has created an asymmetric membrane. Imagine rigid foam with a thin membrane skin. The separation takes place at the skin and the water passes quickly through the foam's large pores. Without the foam, the skin or film layer is not strong enough to withstand the pressure of RO.